This invention relates to a solution polymerization process for the preparation of ethylene propylene elastomers using a comparatively inexpensive single site catalyst system. The catalyst of the present process is an unbridged Group 4 organometallic complex having a cyclopentadienyl ligand and a monosubstituted nitrogen ligand. A boron activator is also required.
Ethylene propylene (EP) elastomers are widely available items of commerce which are prepared by copolymerizing ethylene, propylene and (optimally) a small amount of a diene monomer. Copolymers which contain at least 20 weight % of randomly distributed propylene units are substantially less crystalline than typical thermoplastic polyethylene or polypropylene homopolymers. A combination of low crystallinity and high molecular weight generally provides elastomeric properties in these polymers. These elastomers are used in many applications such as membranes (for roofing or for pond liners); blending components for the preparation of xe2x80x9ctoughedxe2x80x9d thermoplastics (such as xe2x80x9ctoughenedxe2x80x9d polypropylene and toughened nylon) and, in particular, automotive parts. Examples of automotive parts which are made from ethylene propylene elastomers include belts, seals, hoses and tire sidewalls.
Ethylene propylene elastomers may also include a small amount of a diene. This leaves residual unsaturation in the elastomer which may be usefully employed to prepare xe2x80x9cvulcanizedxe2x80x9d or xe2x80x9ccuredxe2x80x9d compounds. Such elastomers are typically referred to as xe2x80x9cEPDMxe2x80x9d.
EP and/or EPDM elastomers generally require a weight average molecular weight (xe2x80x9cMwxe2x80x9d) of at least 60,000 in order to provide sufficient tensile strength for use in automotive applications. These elastomers may be produced in slurry and solution polymerization processes.
Slurry polymerization processes are particularly suitable for preparing extremely high molecular weight ethylene propylene (diene) elastomers.
Solution polymerization processes are somewhat less suitable for the preparation of high molecular weight ethylene propylene (diene) elastomers because the high solution viscosity of high molecular weight elastomers makes such solutions difficult to handle. This problem may be mitigated by increasing the solution temperature. However, the use of higher polymerization temperatures generally increases the rate of chain termination reactions and thereby lowers the molecular weight of the polymer.
Conventional EP and EPDM elastomers are typically prepared with a Ziegler catalyst system comprising a Group 4 or 5 metal and an alkyl aluminum (halide) cocatalyst. Vanadium is the generally preferred metal because it provides elastomers having high molecular weight. Exemplary vanadium compounds include vanadium halides (especially vanadium chloride), vanadium alkoxides and vanadium oxy halides (such as VOCl3). These vanadium compounds are inexpensive but are not particularly active.
More recently, the use of xe2x80x9csingle site catalystsxe2x80x9d such as metallocene catalysts has been proposed for the preparation of EP or EPDM elastomers. These catalysts are generally more expensive than the simple vanadium components described above. In particular, high catalyst costs are incurred due to the cost of synthesizing the organometallic catalyst complexes and/or when large amounts of alumoxane cocatalysts are used. Accordingly, high polymerization activity (as well as the capability to produce high molecular weight EP and EPDM polymers) is required if these new catalysts are to provide economically viable alternatives to the vanadium compounds.
Bridged metallocene catalysts (i.e. catalysts having a bridging group which is bonded to two cyclopentadienyl or indenyl or fluorenyl ligands) have been proposed for the preparation of EP elastomers. See for example, U.S. Pat. No. 4,871,705 (Hoel; to Exxon), U.S. Pat. No. 5,229,478 (Floyld et al.; to Exxon) and U.S. Pat. No. 5,491,207 (Hoel; to Exxon).
The use of bridged metallocene is potentially desirable because such catalysts may be more stable (i.e. less prone to decomposition) than unbridged catalysts under ethylene propylene polymerization conditions. However, bridged metallocenes are comparatively difficult and expensive to synthesize. Moreover, such catalysts can lead to the formation of isotactic polypropylene sequences in ethylene propylene polymers (as disclosed in European Patent Application (EPA) 374,695; Davis et al; to Polysar Ltd.) which is not desirable for products that are intended for use as elastomers.
Similarly, U.S. Pat. No. 5,696,213 (Schiffino et al.; to Exxon) teaches the preparation of EP and EPDM in a solution process using a cyclic monocyclopentadienyl Group 4 metallocene catalyst (i.e. a catalyst having a bridged (or xe2x80x9ccyclicxe2x80x9d) ligand in which the cyclopentadienyl group forms part of the xe2x80x9cbridgexe2x80x9d (or xe2x80x9ccyclicxe2x80x9d) ligand with another atomxe2x80x94such as a group 15 heteroatom being bonded both to the cyclopentadienyl ligand and the Group 4 metal so as to form the rest of the cyclic ligand. This patent also teaches the use of a bridged bis indenyl hafnium catalyst.
A process for the preparation of an elastomeric ethylene-propylene polymer wherein said process is characterized by being undertaken under solution polymerization conditions in the presence of a catalyst system which comprises:
1) an unbridged catalyst having a single cyclopentadienyl ligand and a monosubstituted nitrogen ligand; and
2) a boron activator;
wherein said catalyst is defined by the formula: 
wherein Y is selected from the group consisting of:
ai) a phosphorus substituent defined by the formula: 
wherein each R1 is independently selected from the group consisting of a hydrogen atom, a halogen atom, C1-20 hydrocarbyl radicals which are unsubstituted by or further substituted by a halogen atom, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula:
xe2x80x94Sixe2x80x94(R2)3
wherein each R2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical of the formula:
xe2x80x94Gexe2x80x94(R2xe2x80x2)3
wherein R2xe2x80x2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical; and
aii) a substituent defined by the formula: 
wherein each of Sub1 and Sub2 is independently selected from the group consisting of hydrocarbyls having from 1 to 20 carbon atoms; silyl groups, amido groups and phosphido groups.
Cp is a ligand selected from the group consisting of cyclopentadienyl, substituted cyclopentadienyl, indenyl, substituted indenyl, fluorenyl and substituted fluorenyl;
X is an activatable ligand and n is 1 or 2, depending upon the valence of M and the valence of X; and
M is a group 4 metal selected from the group consisting of titanium, hafnium and zirconium.
Preferred elastomeric polymers have a weight average molecular weight of at least 60,000 and a propylene content of at least 20 weight %.
As noted above, the process of this invention must employ a boron activator. As described later and illustrated in the examples, it is particularly preferred to use a small amount of the activator (especially an equimolar amount of the catalyst and activator). This can provide a cost advantage in comparison to the more conventional use of large molar excesses of alumoxane cocatalyst. In addition, whilst not wishing to be bound by theory, it is believed that large molar excesses of alumoxane may lead to the degradation of the catalysts of this invention under the conditions required for the solution polymerization of ethylene propylene elastomers. (More particularly, it is postulated that large molar excesses of alumoxane may lead to undesirable interactions or reactions with the metal-nitrogen bond of the catalysts of this invention, such as the formation of bridging groups and/or cleavage of the metal-nitrogen bond.)
The catalyst used in the process of this invention is a Group 4 organometallic complex which is characterized by having a cyclopentadienyl ligand, a monosubstituted nitrogen ligand (which is a phosphinimine ligand or a ketimide ligand) and at least one activatable ligand. Each of these ligands is described in detail below.
The catalyst preferably contains a phosphinimine ligand which is covalently bonded to the metal. This ligand is defined by the formula: 
wherein each R1 is independently selected from the group consisting of a hydrogen atom, a halogen atom, C1-20 hydrocarbyl radicals which are unsubstituted by or further substituted by a halogen atom, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula:
xe2x80x94Sixe2x80x94(R2)3
wherein each R2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical of the formula:
xe2x80x94Gexe2x80x94(R2xe2x80x2)3
wherein R2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical.
This ligand contains a xe2x80x9cmonosubstituted nitrogen atomxe2x80x9d in the sense that there is only one phosphorus atom (doubly) bonded to the nitrogen atom.
The preferred phosphinimines are those in which each R1 is a hydrocarbyl radical. A particularly preferred phosphinimine is tri-(tertiary butyl) phosphinimine (i.e. where each R1 is a tertiary butyl group).
As used herein, the term xe2x80x9cketimide ligandxe2x80x9d refers to a ligand which: (a) is bonded to the transition metal via a metal-nitrogen atom bond; (b) has a single substituent on the nitrogen atom, (where this single substituent is a carbon atom which is doubly bonded to the N atom); and (c) has two substituents (Sub 1 and Sub 2, described below) which are bonded to the carbon atom.
Conditions a, b and c are illustrated below: 
This ligand also contains a monosubstituted nitrogen atom in the sense that only one carbon atom is (doubly) bonded to the nitrogen atom.
The substituents xe2x80x9cSub 1xe2x80x9d and xe2x80x9cSub 2xe2x80x9d may be the same or different. Exemplary substituents include hydrocarbyls having from 1 to 20 carbon atoms; silyl groups, amido groups and phosphido groups. For reasons of cost and convenience it is preferred that these substituents both be hydrocarbyls, especially simple alkyls and most preferably tertiary butyl.
As used herein, the term cyclopentadienyl ligand is meant to broadly convey its conventional meaning, namely a ligand having a five carbon ring which is bonded to the metal via eta-5 bonding. Thus, the term xe2x80x9ccyclopentadienylxe2x80x9d includes unsubstituted cyclopentadienyl, substituted cyclopentadienyl, unsubstituted indenyl, substituted indenyl, unsubstituted fluorenyl and substituted fluorenyl. An exemplary list of substituents for a cyclopentadienyl ligand includes the group consisting of C1-10 hydrocarbyl radical (which hydrocarbyl substituents are unsubstituted or further substituted); a halogen atom, C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical; an amido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; silyl radicals of the formula xe2x80x94Sixe2x80x94(R)3 wherein each R is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical C6-10 aryl or aryloxy radicals; germanyl radicals of the formula Gexe2x80x94(R)3 wherein R is as defined directly above.
The catalyst used in the process of this invention must also contain an activatable ligand. The term xe2x80x9cactivatable ligandxe2x80x9d refers to a ligand which may be activated by the boron activator (or a combination of the boron activator with a small amount of alumoxane) to facilitate olefin polymerization. Exemplary activatable ligands are independently selected from the group consisting of a hydrogen atom, a halogen atom, a C1-10 hydrocarbyl radical, a C1-10 alkoxy radical, a C5-10 aryl oxide radical; each of which said hydrocarbyl, alkoxy, and aryl oxide radicals may be unsubstituted by or further substituted by a halogen atom, a C1-8 alkyl radical, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals.
The number of activatable ligands depends upon the valency of the metal and the valency of the activatable ligand. The preferred catalyst metals are Group 4 metals in their highest oxidation state (i.e. 4+) and the preferred activatable ligands are monoanionic (such as a hydrocarbyl groupxe2x80x94especially methyl). Thus, the preferred catalyst contain a phosphinimine ligand, a cyclopentadienyl ligand and two chloride (or methyl) ligands bonded to the Group 4 metal. In some instances, the metal of the catalyst component may not be in the highest oxidation state. For example, a titanium (III) component would contain only one activatable ligand.
Both of the xe2x80x9cphosphiniminexe2x80x9d and xe2x80x9cketimidexe2x80x9d ligands have been discovered to provide high molecular weight EP and/or EPDM elastomers under solution polymerization conditions with surprisingly good activity when activated with a boron activator. The xe2x80x9cphosphiniminexe2x80x9d catalysts are preferred because of their particularly good activity and for reasons which will be apparent upon consideration of the data in the Examples. More particularly, the most preferred catalysts are Group 4 organometallic complex in its highest oxidation state having a phosphinimine ligand, a cyclopentadienyl-type ligand and two activatable ligands. These requirements may be concisely described using the following formula for the preferred catalyst: 
wherein: (a) M is a metal selected from Ti, Hf and Zr; (b) Pl is a phosphinimine ligand defined by the formula: 
wherein each R1 is independently selected from the group consisting of a hydrogen atom, a halogen atom, C1-20 hydrocarbyl radicals which are unsubstituted by or further substituted by a halogen atom, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula:
xe2x80x94Sixe2x80x94(R 2)3
wherein each R2xe2x80x2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical of the formula:
xe2x80x94Gexe2x80x94(R2xe2x80x2)3
wherein R2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical; (c) Cp is a ligand selected from the group consisting of cyclopentadienyl, substituted cyclopentadienyl, indenyl, substituted indenyl, fluorenyl, substituted fluorenyl; and (d) X is an activatable ligand, and wherein: m is 1, n is 1 and p is 2.
The boron activators used in this invention, also referred to those skilled in the art as, xe2x80x9cionic activatorsxe2x80x9d and are well known for use with metallocene catalysts. See, for example, U.S. Pat. No. 5,198,401 (Hlatky and Turner) and U.S. Pat. No. 5,132,380 (Stevens and Neithamer).
Whilst not wishing to be bound by any theory, it is thought by those skilled in the art that xe2x80x9cionic activatorsxe2x80x9d initially cause the abstraction of one or more of the activatable ligands in a manner which ionizes the catalyst into a cation, then provides a bulky, labile, non-coordinating anion which stabilizes the catalyst in a cationic form. The bulky, non-coordinating anion coordinating anion permits olefin polymerization to proceed at the cationic catalyst center (presumably because the non-coordinating anion is sufficiently labile to be displaced by monomer which coordinates to the catalyst). Preferred boron activators are described in (i)-(iii) below:
(i) compounds of the formula [R5]+[B(R7)4]xe2x88x92 wherein B is a boron atom, R5 is a aromatic hydrocarbyl (e.g. triphenyl methyl cation) and each R7 is independently selected from the group consisting of phenyl radicals which are unsubstituted or substituted with from 3 to 5 substituents selected from the group consisting of a fluorine atom, a C1-4 alkyl or alkoxy radical which is unsubstituted or substituted by a fluorine atom; and a silyl radical of the formula xe2x80x94Sixe2x80x94(R9)3 wherein each R9 is independently selected from the group consisting of a hydrogen atom and a C1-4 alkyl radical; and
(ii) compounds of the formula [(R8)t ZH]+[B(R7)4]xe2x88x92 wherein B is a boron atom, H is a hydrogen atom, Z is a nitrogen atom or phosphorus atom, t is 2 or 3 and R8 is selected from the group consisting of C1-8 alkyl radicals, a phenyl radical which is unsubstituted or substituted by up to three C1-4 alkyl radicals, or one R8 taken together with the nitrogen atom may form an anilinium radical and R7 is as defined above; and
(iii) compounds of the formula B(R7)3 wherein R7 is as defined above. [Note: B(R7)3 is not an ionic compound. Whilst not wishing to be bound by theory, it is believed that compounds of the formula B(R7)3 abstract an activatable ligand (L) from the catalyst species, thus forming a non coordinating anion of the formula [B(R7)3L]xe2x88x92 wherein L is an activatable ligand as previously described herein.
In the above compounds preferably R7 is a pentafluorophenyl radical, R5 is a triphenylmethyl cation, Z is a nitrogen atom and R8 is a C1-4 alkyl radical or R8 taken together with the nitrogen atom forms an anilinium radical which is substituted by two C14 alkyl radicals.
Whilst not wishing to be bound by theory, it is postulated that the boron activator may abstract one or more activatable ligands so as to ionize the catalyst center into a cation but not to covalently bond with the catalyst and to provide sufficient distance between the catalyst and the ionizing activator to permit a polymerizable olefin to enter the resulting active site.
Examples of boron activators include:
triethylammonium tetra(phenyl)boron,
tripropylammonium tetra(phenyl)boron,
tri(n-butyl)ammonium tetra(phenyl)boron,
trimethylammonium tetra(p-tolyl)boron,
trimethylammonium tetra(o-tolyl)boron,
tributylammonium tetra(pentafluorophenyl)boron,
tripropylammonium tetra(o,p-dimethylphenyl)boron,
tributylammonium tetra(m,m-dimethylphenyl)boron,
tributylammonium tetra(p-trifluoromethylphenyl)boron,
tributylammonium tetra(pentafluorophenyl)boron,
tri(n-butyl)ammonium tetra(o-tolyl)boron,
N,N-dimethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)n-butylboron,
N,N-2,4,6-pentamethylanilinium tetra(phenyl)boron,
di-(isopropyl)ammonium tetra(pentafluorophenyl)boron,
dicyclohexylammonium tetra(phenyl)boron,
triphenylphosphonium tetra(phenyl)boron,
tri(methylphenyl)phosphonium tetra(phenyl)boron,
tri(dimethylphenyl)phosphonium tetra(phenyl)boron,
tropillium tetrakispentafluorophenyl borate,
triphenylmethylium tetrakispentafluorophenyl borate,
benzene(diazonium)tetrakispentafluorophenyl borate,
tropillium phenyltrispentafluorophenyl borate,
triphenylmethylium phenyltrispentafluorophenyl borate,
benzene(diazonium)phenyltrispentafluorophenyl borate,
tropillium tetrakis(2,3,5,6-tetrafluorophenyl)borate,
triphenylmethylium tetrakis(2,3,5,6-tetrafluorophenyl)borate,
benzene(diazonium)tetrakis(3,4,5-trifluorophenyl)borate,
tropillium tetrakis(3,4,5-trifluorophenyl)borate,
benzene(diazonium)tetrakis(3,4,5-trifluorophenyl)borate,
tropillium tetrakis(1,2,2-trifluoroethenyl)borate,
triphenylmethylium tetrakis(1,2,2-trifluoroethenyl)borate,
benzene(diazonium)tetrakis(1,2,2-trifluoroethenyl)borate,
tropillium tetrakis(2,3,4,5-tetrafluorophenyl)borate,
triphenylmethylium tetrakis(2,3,4,5-tetrafluorophenyl)borate, and
benzene(diazonium)tetrakis(2,3,4,5-tetrafluorophenyl)borate.
Commercially available boron activators include: N,N-dimethylaniliniumtetrakispentafluorophenyl borate, triphenylmethylium tetrakispentafluorophenyl borate, and trispentafluorophenyl borane.
The boron activator is preferably added to the reactor on a roughly equimolar basis to the transition metal of the catalyst. Mole ratios of from 0.5/1 to 2/1 may be used, with 1/1 to 1.2/1 being especially preferred. It would be permissible (but wasteful and expensive) to use large molar excesses of the boron activator.
Alumoxanes may not be used as the sole cocatalyst in the process of this invention (because of the comparatively poor activity under ethylene propylene solution polymerization conditions, as shown in the Examples). However, alumoxanes may be used as a (second) cocatalyst and/or as a poison scavenger.
The alumoxane may be of the formula:
(R4)2AlO(R4AlO)mAl(R4)2
wherein each R4 is independently selected from the group consisting of C1-20 hydrocarbyl radicals and m is from 0 to 50, preferably R4 is a C1-4 alkyl radical and m is from 5 to 30. Methylalumoxane (or xe2x80x9cMAOxe2x80x9d) in which each R is methyl is the preferred alumoxane.
Alumoxanes are also readily available articles of commerce generally as a solution in a hydrocarbon solvent.
The alumoxane, when employed, is preferably added at an aluminum to transition metal (in the catalyst) mole ratio of from 20:1 to 1000:1. Preferred ratios are from 5:1 to 250:1.
Furthermore, whilst not wishing to be bound by any theory, it is believed that the metal-nitrogen bond of the catalysts of this invention may be susceptible to degradation/cleavage by acidic poisons under the polymerization conditions of this invention. These poisons may be present in the solvent, monomers or even in the alumoxane solution (such as residual alkyl aluminum). Accordingly, it is also preferred to use a minor amount of a base as a scavenger for these poisons. It is particularly preferred that this base be sterically bulky. Sterically bulky amines and/or sterically bulky alcohols are preferred.
The data provided in the Examples show a surprising activity increase when an alumoxane is used in combination with the boron activator (in comparison to the boron above, and in particular, in comparison to the poor activity obtained when the alumoxane is used above). However, it has not been conclusively established whether this desirable result is caused by the catalyst-alumoxane xe2x80x9cactivatingxe2x80x9d influence, or from the mitigation of the deleterious effects of catalyst poisons or some combination thereof.
Description of Solution Polymerization Process
Solution processes for the polymerization of ethylene propylene elastomers are well known in the art. These processes are conducted in the presence of an inert hydrocarbon solvent such as a C5-12 hydrocarbon which may be unsubstituted or substituted by a C1-4 alkyl group such as pentane, methyl pentane, hexane, heptane, octane, cyclohexane, methylcyclohexane and hydrogenated naphtha. An example of a suitable solvent which is commercially available is xe2x80x9cIsopar Exe2x80x9d (C8-12 aliphatic solvent, Exxon Chemical Co.).
The process of this invention is undertaken at a temperature of 10xc2x0 C. to 150xc2x0 C., such as 20xc2x0 C. As previously noted, the use of a high polymerization temperature will generally reduce the solution viscosity (which is desirable) but also reduce molecular weight (which may be undesirable). The preferred polymerization temperature is less than 100xc2x0 C., where a surprising combination of excellent polymerization activity and excellent molecular weight may be obtained.
The present invention is a process which is used to prepare elastomer co- and ter-polymers of ethylene, propylene and optionally one or more diene monomers. Generally, such polymers will contain about 50 to about 80 weight % ethylene (preferably about 50 to 60 weight % ethylene) and correspondingly from 50 to 20 weight % of propylene. The elastomers of this invention may also be prepared with a small amount of diene monomer so as to facilitate crosslinking or vulcanization of the elastomerxe2x80x94as is well known to those skilled in the art. The diene is preferably present in amounts up to 10 weight % of the polymer and most preferably is present in amounts from about 3 to 7 weight %. The resulting polymer may have a composition comprising from 40 to 75 weight % of ethylene, from 50 to 15 weight % of propylene and up to 10 weight % of a diene monomer to provide 100 weight % of the polymer. More than one type of diene monomer may be included. Preferred but not limiting examples of the dienes are dicyclopentadiene, 1,4-hexadiene, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene and 5-vinyl-2-norbornene, especially 5-ethylidene-2-norbornene and 1,4-hexadiene.
The monomers are dissolved/dispersed in the solvent either prior to being fed to the reactor (or for gaseous monomers the monomer may be fed to the reactor so that it will dissolve in the reaction mixture). Prior to mixing, the solvent and monomers are generally purified to remove potential catalyst poisons such as water, oxygen or metal impurities. The feedstock purification follows standard practices in the art, e.g. molecular sieves, alumina beds and oxygen removal catalysts are used for the purification of monomers. The solvent itself as well (e.g. methyl pentane, cyclohexane, hexane or toluene) is preferably treated in a similar manner.
The feedstock may be heated or cooled prior to feeding to the polymerization reactor. Additional monomers and solvent may be added to the second reactor (if employed) and it may be heated or cooled.
Generally, the catalyst components may be premixed in the solvent for the reaction or fed as separate streams to each reactor. In some instances premixing it may be desirable to provide a reaction time for the catalyst components prior to entering the reaction. Such an xe2x80x9cin line mixingxe2x80x9d technique is described in a number of patents in the name of DuPont Canada Inc. (e.g. U.S. Pat. No. 5,589,555, issued Dec. 31, 1996).
The residence time in the polymerization reactor will depend on the design and the capacity of the reactor. Generally the reactors should be operated under conditions to achieve a thorough mixing of the reactants. If a dual reactor polymerization process is employed, it is preferred that from 20 to 60 weight % of the final polymer is polymerized in the first reactor, with the balance being polymerized in the second reactor. On leaving the reactor the solvent is removed and the resulting polymer is finished in a conventional manner.
It is also within the scope of this invention to use more than two polymerization reactors.